1. Field of the Invention
The present invention relates to a three parallel shaft type automatic transmission in which a first input shaft, a second input shaft adapted to rotate in synchronism with the first input shaft and an output shaft are disposed in parallel with one another.
2. Description of the Related Art
In general, a parallel shaft type automatic transmission includes two shafts, an output shaft and an input shaft, which are disposed in parallel with each other and a plurality of speed gears provided in series between the input shaft and the output shaft via hydraulic clutches provided individually for the speed gears, respectively. With this parallel shaft type automatic transmission, however, if the number of speed gears is increased, the axial length of the transmission is increased. To cope with this, there is also conventionally known a three parallel shaft type automatic transmission in which a first input shaft, a second input shaft adapted to rotate in synchronism with the first input shaft and an output shaft are disposed in parallel with one another, and in which a plurality of speed gears are provided in series between the first and second input shafts and the output shaft, respectively.
In addition, the size of the transmission has to be increased if a hydraulic clutch is provided exclusively for the reverse gear. Therefore, with a view to sharing the hydraulic clutches (the first hydraulic clutch) for obtaining any of the forward gears (the first gear) disposed on the first input shaft for use for the reverse gear, there is known an automatic transmission disclosed in JP-A-10-103463. Specifically, a reverse drive gear is integrally connected to a first speed drive gear which is connected to the first input shaft via a first hydraulic clutch and is rotatably supported on a first input shaft. In an output shaft, a first speed driven gear for mesh engagement with the first speed drive gear and a reverse driven gear which meshes with the reverse drive gear via an idle gear are positioned at axial ends of a selector hub connected to an output shaft, and are rotatably supported on the output shaft. And, both the driven gears are made free to be selectively connected to the selector hub via a forward and reverse switching selector. When the first hydraulic clutch is engaged in a state in which the first speed driven gear is connected to the selector hub, a first gear is obtained. On the other hand, when the first hydraulic clutch is engaged in a state in which the reverse driven gear is connected to the selector hub, a reverse gear is obtained.
Additionally, in the automatic transmission constructed as described above, a forward second gear is disposed between the second input shaft and the output shaft so as to be obtained by the engagement of the second hydraulic clutch disposed on the second input shaft on the same side as the first hydraulic clutch with respect to the axial direction. A second speed driven gear is connected to the output shaft while being kept contiguous with the reverse driven gear.
In the conventional automatic transmission, the reason why the second speed driven gear is disposed contiguous with the reverse driven gear is to reduce the axial length of the transmission as much as possible. In this particular conventional automatic transmission, however, when the first gear is obtained, the reverse driven gear rotates in a reverse direction relative to the second speed driven gear which rotates in the normal direction together with the output shaft due to power transmission from the first hydraulic clutch. This is to generate friction between the second speed driven gear and the reverse driven gear while the vehicle is running in the first gear, this resulting in increase in friction loss.
The present invention was made in view of the aforesaid problem and an object thereof is to provide a three parallel shaft type automatic transmission which can make its axial length as short as possible and reduce the friction loss.
With a view to attaining the object, according to the invention, there is provided a three parallel shaft type automatic transmission, including a first input shaft, a second input shaft adapted to rotate in synchronism with the first input shaft and an output shaft which are disposed in parallel with one another. A first hydraulic clutch for obtaining a forward first gear and a second hydraulic clutch for obtaining a forward second gear are separately provided on the first input shaft and the second input shaft, respectively. A drive gear for the first gear connected to the first input shaft via the first hydraulic clutch and a reverse drive gear integrally connected to the first gear drive gear are rotatably supported on the first input shaft. A driven gear for the first gear for mesh engagement with the first gear drive gear and a reverse driven gear adapted to mesh with the reverse drive gear via an idle gear are positioned on axial ends of a selector hub connected to the output shaft to be rotatably supported on the output shaft, both the first gear driven gear and reverse driven gear being made free to be selectively connected to the selector hub via a forward and reverse switching selector. A drive gear for the second gear connected to the second input shaft via the second hydraulic clutch is rotatably supported on the second input shaft, whereas a driven gear for the second gear adapted to mesh with the second gear drive gear is connected to the output shaft. The second gear driven gear is disposed contiguously with the first gear driven gear, a spacer collar is disposed on the output shaft so as to be positioned between the selector hub and the second gear driven gear, and the first gear driven gear is rotatably supported on the spacer collar.
According to the invention, there is generated no friction between the second gear driven gear and the reverse driven gear when the first gear is obtained. On the other hand, when the reverse gear is obtained, the first gear driven gear rotates in the normal direction through transmission of power thereto from the first hydraulic clutch relative to the second gear driven gear which rotates in the reverse direction together with the output shaft, thereby producing the possibility that there is caused friction between the first gear driven gear and the second gear driven gear. According to the invention, however, the press contact of the second gear driven gear against the first gear driven gear is avoided by means of the spacer collar, and therefore friction loss that would be caused by friction generated between both the driven gears does not become so large. In addition, even if there is generated a slight amount of friction, there is extremely low in probability that the reverse gear is obtained, and therefore there is no risk of the fuel economy being deteriorated by the friction between the two driven gears.
Incidentally, the drive gear and driven gear for each speed gear have helical teeth. Then, in the event that the orientation of helical teeth formed in the first gear driven gear is set so as to generate a thrust force in a direction in which the driven gear approaches the selector hub when the first gear driven gear rotates in the normal direction which is a forward rotating direction of the driven gear, when the reverse gear is obtained, the first gear driven gear is pressed by the thrust force in a direction in which the first gear driven gear is separated away from the second gear driven gear, no friction being generated thereby between the two driven gears.
On the other hand, since the first gear driven gear is pressed against the selector hub by the thrust force, there is caused friction between the first gear driven gear and the selector hub. However, since slide contact surfaces of the two members have oil grooves originally formed for supplying lubricating oil to the selector and are sufficiently lubricated, friction loss, if any, between the first gear driven gear and the selector hub becomes extremely small. In addition, in the event that the orientation of the helical teeth formed in the first gear driven gear is set so as to generate a thrust forth in a direction in which the first gear driven gear is separated apart from the selector hub when the same driven gear rotates in the clockwise direction, since there is caused friction between the first gear driven gear and the second gear driven gear when the reverse gear is obtained, there needs to form oil grooves in the second gear driven gear for lubricating the slide contact surface thereof which confronts the first gear driven gear. Then, since the formation of the oil grooves reduces the effective length of a connecting portion of the second gear driven gear relative to the output shaft by the depth of the oil grooves, the overall length of the connecting portion has to be increased with a view to ensuring an effective length required for maintaining a predetermined strength, this calling a problem that the axial length of the transmission is increased. To cope with this, in the event the orientation of the helical teeth formed in the first gear driven gear is set so as to generate the thrust force in the direction in which the first gear driven gear approaches the selector hub when the first gear driven gear rotates in the normal direction as described above, there needs to form no oil grooves for lubricating oil in the second gear driven gear, the aforesaid drawback is not generated.
Note that in an embodiment which will be described below the aforesaid first gear denotes a 4th speed G4, and the second gear a third speed G3.